Re: [PATCH v3 0/5] Dump off-cpu samples directly

[Arnaldo Carvalho de Melo]

On Wed, Jul 31, 2024 at 10:46:18AM -0700, Namhyung Kim wrote:
> On Mon, Jul 29, 2024 at 11:24:47PM +0800, Howard Chu wrote:
> > On Mon, Jul 29, 2024 at 9:21 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > > On Fri, Jul 26, 2024 at 06:28:21PM +0800, Howard Chu wrote:
> > > > As mentioned in: https://bugzilla.kernel.org/show_bug.cgi?id=207323

> > > > Currently, off-cpu samples are dumped when perf record is exiting. This
> > > > results in off-cpu samples being after the regular samples. This patch
> > > > series makes possible dumping off-cpu samples on-the-fly, directly into
> > > > perf ring buffer. And it dispatches those samples to the correct format
> > > > for perf.data consumers.

> > > Thanks for your work!

> > > But I'm not sure we need a separate event for offcpu-time-direct.  If we
> > > fix the format for the direct event, we can adjust the format of offcpu-
> > > time when it dumps at the end.

> > Thank you and Ian for this advice, I'll do that.

> > > Anyway, as far as I can see you don't need to fill the sample info in
> > > the offcpu-time-direct manually in your BPF program.  Because the
> > > bpf_perf_event_output() will call perf_event_output() which fills all
> > > the sample information according to the sample_type flags.

> > > Well.. it'll set IP to the schedule function, but it should be ok.
> > > (updating IP using CALLCHAIN like in off_cpu_write() is a kinda hack and
> > > not absoluately necessary, probably I can get rid of it..  Let's go with
> > > simple for now.)

> > > So I think what you need is to ensure it has the uncessary flags.  And
> > > the only info it needs to fill is the time between the previous schedule
> > > and this can be added to the raw data.

I wonder if there wouldn't be other kernel information about things that
may have affected the time it took for the task to be off-cpu, maybe
system load average, C/P states, but then it would be overengineering, I
think, just thought about what else we could add that could help
understanding the off-cpu time that could be obtained from the vantage
point of BPF attached to sched-out and sched-in, things we could collect
at sched-out in addition to the timestamp and ditto at sched-in, but I'm
no scheduler expert, so take this just as some brainstorming. Maybe we
could have some sort of sample_type specific to this off-cpu event that
would allow us to ask for extra info in an extensible way. We can start
with just PERF_OFFCPU_SAMPLE_TIMESTAMP...

> > Sure thing, thank you. Other than the off-cpu duration, do you think
> > we should collect the callchain as well?
 
> I think the kernel will do that for you once you set the
> SAMPLE_CALLCHAIN flag in the event attr.

Yes, we should not reinvent the wheel for all things that can be asked
from the kernel perf subsystem, only using the raw-data payload on the
bpf-output event for things we can't get from the perf subsystem, and
that is the timestamp for a previous event stored in a BPF map, looking
if the delta to the current time (on a sched-in event) is over the
threshold and then recording this time on this specific "made-up on the
fly using BPF" event that appears on the ring buffer just like any other
"native" events such as tracepoints, hardware events, cache events, etc.

- Arnaldo